This invention relates generally to current transformers and more particularly to current transformers used in ground fault circuit breakers.
Ground fault circuit breakers for alternating current distribution circuits are commonly used to protect people against dangerous shocks due to line-to-ground current flow through someone""s body. Ground fault circuit breakers must be able to detect current flow between line conductors and ground at current levels as little as 5 milliamperes, which is much below the overload current levels required to trip conventional circuit breakers. Upon detection of such a ground fault current, the contacts of the circuit breaker are opened to deenergize the circuit.
Current transformers are an integral part of ground fault circuit breakers in that such circuit breakers typically include two of the transformers. A first current transformer, referred to as the ground fault or sense transformer, is used to sense ground fault currents. The sense transformer has as its primary windings the conductors of the distribution circuit being protected, which are encircled by the core, and a multi-turn winding wound on the core. (In the case of a one pole breaker, the line and neutral conductors both go through the sense transformer core, and in the case of a two pole breaker, the two line conductors and the neutral conductor all go through this core. For the sake of example, the following discussion relates to a one pole breaker.) During normal conditions, the current flowing in one direction through the line conductor will return in the opposite direction through the neutral conductor. This produces a net current flow of zero through the transformer, and the multi-turn winding provides no output. However, if a fault (that is, a leakage path) is established between the line conductor and ground, return current will bypass the transformer and flow through the ground back to the grounded side of the source supplying the circuit. Thus, more current will be flowing in one direction through the transformer than in the other, producing a current imbalance. Such a current imbalance produces uncancelled flux in the sense transformer""s core, resulting in an output from the multiturn winding that trips the circuit breaker mechanism.
A second current transformer, referred to as the ground neutral transformer, is commonly used to detect neutral-to-ground faults. A neutral-to-ground fault is an inadvertent short between the neutral conductor and ground that may occur due to a fault such as a wiring error by the electrician installing the circuit breaker. Such a leakage path on the load side of the sense transformer does not in itself produce a shock hazard; however, the occurrence of a grounded neutral at the same time as a ground fault on a line conductor will cause the ground fault circuit breaker to be less sensitive in detecting ground fault currents, thereby creating a hazardous situation. A neutral-to-ground fault reduces the sensitivity of the sense transformer as a ground fault sensing device because such a fault tends to provide a return current path via the neutral conductor for a large portion of the line-to-ground leakage current. To the extent that line-to-ground leakage current returns to the source via the neutral conductor, it escapes detection by the sense transformer. Consequently, the sense transformer may not respond to a hazardous ground fault.
In one known application, the ground neutral transformer comprises a core that encircles the neutral conductor (the ground neutral core can, but need not, encircle the line conductor too) and has a multi-turn winding wound thereon. When a neutral-to-ground fault occurs, an inductively coupled path between the sense transformer and the ground neutral transformer is closed. The resultant coupling produces an output in the ground fault sense transformer that trips the circuit breaker mechanism.
Such circuit breakers provide generally satisfactory operation. However, because of a current transformer""s finite permeability, a dipolar asymmetry in the magnetic properties of the transformer""s core and/or multi-turn winding will exist if the conductors are not symmetrically located in the opening of the transformer. The sense transformer of a ground fault circuit breaker must be able to detect a current imbalance as little as 5 milliamperes in the presence of hundreds of amperes of current. Thus, even a small dipolar asymmetry can produce an unacceptable error that will degrade the sense transformer""s ability to detect ground fault currents.
Conventional current transformers often address this problem with magnetic shielding around the core, but magnetic shielding adds considerable cost to the current transformer. Magnetic shielding also increases the volume of the transformer. This can be a problem in ground fault circuit breakers because it can be difficult to package two transformers, the large #12 or #14 conductors, and a printed circuit board (which contains standard circuit breaker circuitry), into the small allotted volume provided in existing circuit breaker housings. This is particularly the case in residential applications for which compact, half-inch circuit breakers are now available.
It is also known to use high saturation core materials, such as those available under the trademark Permalloy, to minimize the dipolar asymmetry. However, such materials are typically more expensive than other common core materials such as ferrite.
Accordingly, there is a need for a current transformer that provides accurate output without using magnetic shielding or expensive materials.
The above-mentioned need is met by exemplary embodiments of the present invention which provide a current transformer for a ground fault circuit breaker used on a circuit having one or more line conductors and a neutral conductor. The current transformer includes a toroidal core having a circular opening defining a center point and a multi-turn winding wound on the core. A first guide member is disposed on one side of the core, and a second guide member is disposed on another side of the core. The first and second guide members each have a hole for receiving the line conductor and a hole for receiving the neutral conductor formed therein. The guide members thus position the conductors with respect to the core. In addition, a method of correcting asymmetries in the current transformer is provided. The method includes measuring the magnitude and orientation of any asymmetries, and then altering the current transformer based on the measured magnitude and orientation of the asymmetries so as to eliminate the asymmetries.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.